Acoustic device



Dec. 28, 1948; E. BRUCE 2,457,527 l ACOUSTIC DEVICE Filed Oct. 2, 1942FIG.

' IIIIIIIIIIII/I/Illll IIIIIIIIIIIIIIIIIIII FIG. 2

' wvmrm By E. BRUCE Muzak 151M Patented Dec. 28, 1948 ACOUSTIC DEVICEEdmond Bruce, Red Bank,'N.- J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application October 2, 1942,Serial No; 460,545

This invention relates to acoustic devices and more particularlytoacoustic receivers andtransmitters having marked directionalcharacteristics.

One object of this invention is to obtainahighlydireetionalcharacteristic with a. sound translating device of relatively simpleconstruction and relatively small dimensions.

Another object of this invention is to obtain a high amplitude level ofresponse or propagation for directional acoustic devices.

Directivity and amplitude of response or propagation of soundtranslating devices are basically a matter of superposition ofamplitudes and their phases. Zonal theory of Wave motion indicates thatthe amplitude of pressure at a point in space, due to a plane-wave ofsound, is the sum of the eilects of incremental pressures in a series ofconcentric interference zones located in a plane contained in anyequiphase wave front, intermediate the point and the wave. The eflectofeach zone upon the point in question is dependent, in both amplitude andphase, upon the distance between the zone and the point. In general,theeffect, in amplitude, decreases as this distance increases. As tophase, the zones maybe considered as divided into two groups, the zonesbeing spaced, from the point in question, distances a function of thelength of the propagated wave such that at the point in question thepressures due to each group are in phase; with the pressures: due to thezones in one group in phase opposition to the pressures due to the zonesin the other group.

The amplitude of pressure at the point of interest, due to the centralzone is greater than, theoretically twice as great as, the-sum of theamplitude due to alt the: other zones, considering all zonesto infinity;If this central; zone is considered numerically as the. first zone, theoddnumbered zones constitute useful zones,:. that. is they add totheamplitude due to the central zone, whereas the even-numbered zonesconstitute harm-fur zones. inasmuch as their efiectat. .the point ofinterest'lsin opposition, i. e. out ofphase' with, the pressure due tothe central. and odd numbered zones- In accordance with. a feature ofthis: invention, a sound translating device. is: constructed as a zonal;system: with the receiver or transmitter mounted at the focal point ofthe system and means'are provided for eliminating the efiects oftheharmiul zones, whereby large amplitudes accompanied by markeddirectional characteristics are at the focal point. This means maybeelfectlveto block. out. the effects of the harmful zones, or efiectiveto produce a phase reversal. wherebythese; zones are: self-canceling ineflcct.

16 Claims. (01. 181-31) which:

Fig.v 1 is a sectional view of a .sound translating device illustrativeof one embodiment of this invention wherein the transmitter is subjectedonly to the effects of the central zone;

, Fig. 1A is a front view, to a reduced scale, of the sound translatindevice illustrated in Fig. 1;

Fig. 2 is a graph illustrating the directional characteristic of thedevice shown in Fig. 1;

Fig. 3 is a side view in section of a device i1lus-. trative of anotherembodiment of this invention wherein a number of zones are employed andthe second zone, a harmful zone, is blocked out; and

Fig. 4 is a sectional view of a device illustrative of anotherembodiment of this invention wherein a phase reversal of the pressuresdue to the harmfulzones is produced whereby the harmful zones areself-canceling in effect.

Referring now to the drawing, the sound translating device illustratedinFig. 1 comprises a circular cylindrical drum or shield Ill, forexample of metal, having a plane reflecting base I I. Within the drum I0is an acoustic transmitter or microphone II, which is mounted on theaxis of. the drum and mid-way between the base II and the openendthereof. The drum or shield is dimensioned so that the transmitter ormicrophone is subjected only to the efiects of the first or centraldiffraction zone and of the corresponding reflection zone. Specifically,the device being adapted to translate signals within a band offrequencies, the shield or drum is of. a diameter of substantially1.73X, where i is the mean Wave-length of the signals in the band to betranslated, and is of, a depth or length of approximately i. Thediameter, it will be noted, corresponds to that of the first or centralzone. The distance between any point in the plane of the open endof thedrum and the microphone, considering the depth of the drum or shield ask, lies between and so that the effect of the incremental pressures atall those points is cumulative upon'the microphone. The surface of thebase wall II toward the microphone constitutes a second equiphase wavefront and the effects of all points thereon: upon the microphone alsoare cumulative and'in phase with those of the equiphase wave front in.the plane-of the open endv of the drum.

the microphone is mounted, is several times that which would be producedin the absence of the shield or drum.

Theoretically, the depth or length of the drum should be exactly 1wave-length. However, practically, because of edge effects, a departurefrom the theoretical value is necessary for ptimum operation. In aparticular construction, a depth or length of 1; wave-length was foundto be satisfactory.

The microphone l2 may face either way, that is toward the base II ortoward the open end of the shield or drum l0. Also, it may be of any oneof a number of types and in general should be of small dimensions. Amoving coil type microphone of substantially spherical configuration hasbeen found particularly satisfactory.

A typical directional pattern for an actual device such as illustratedin Fig. I constructed in accordance with this invention is shown in Fig.2, wherein the response is plotted against the angle between the axis ofthe device and the direction of the sound source. It will be noted fromFig. 2 that the device is highly responsive to waves traveling towardthe open end of the drum l0 and substantially parallel to the axis ofthe device and within an angle of approximately 25 degrees to this axison both sides thereof but is only slightly responsive to wavesapproaching the device in directions outside of the angular range noted.

The device illustrated in Fig. 3 is similar to that shown in Fig. 1 anddescribed hereinabove but is of such construction that the microphone I2is subjected to the effects of two useful zones, namely the central andthird zones, and the second zone is blocked out. The circular,cylindrical drum wall I0 is of a diameter of approximately 3.87wave-lengths of the mean frequency in the band of signals to betranslated and the drum length or depth is approximately 1 wave-length,the microphone I! being mounted mid-way between the base I I and theopen end of the drum. Mounted in the plane of the open end of the drumor shield is an annular masking or diffraction member I3, the innerdiameter of which is substantially 1373i and the outer diameter of whichis substantially 2.82x. This diffraction member l3, it will be noted,blocks out the second zone in the plane of the open end of the drum ll!and blocks out also the corresponding reflecting zone on the innersurface of the base II. The opening in the member l3 corresponds to thecentral zone and the opening between the outer edge of the member l3 andthe drum l0 corresponds to the third zone. The distance to the focalpoint, at which the microphone is mounted, from any point in the centralzone is, for a depth of the drum or shield of A, between and i and thedistance between the focal point and any point in the third zone isbetween /l and 2). so that the effects of these zones and also thecorresponding reflecting zones on the wall II, are additive upon themicrophone and a high amplitude level at the microphone together with amarked directional characteristic are realized.

Although the device shown in Fig. 3 is a threezone device it will beunderstood that a greater number of zones can be employed, theevennumbered zones being blocked out by annular masking or diffractionmembers.

In Figs. 1 and 3, described above, the focal length has been consideredas \/2. However, the distance from the microphone I2 to the center ofthe central zone may be any integral multiple oi X/Z. For such distancesof other than the particular value of /\/2 in the specific devicesconsidered hereinabove, the diameters of the several zones will differfrom the particular values, e. g., approximately 1.7x for the centralzone and approximately 2.8 for the second zone, given for the case wherethe distance is approximately A/Z. The general condition to be fulfilledin any case is that the difference between the distance from theboundary of the central zone to the microphone and the distance betweenthe microphone and the center of this zone be approximately M? and thedifference in the distances between the microphone and the boundaries ofany other zone also be M2. Thus, for example, if the microphone isspaced from the center of the central zone, the distance from themicrophone to the boundary of this zone, i. e., to the boundary or theaperture in the plate 13 in Fig. 3 and to the edge boundary of the openend of the shield or drum Ill in big. I, should be approximately (n+l)./2 and the diameter of the central zone, then, should be approximatelyIn the devices illustrated in Figs. 1 and 3 it will be noted that bothdiffraction and reflection Zone means are employ-ed. The invention maybe embodied also in devices where only diffraction zone plates or onlyreflection zone plates are utilized. In such cases, the restrictions onfocal length present in devices employing both reflection anddiffraction zone members, occasioned by the requisite phase relationbetween the effects of the two, are not present. Thus, in devicesemploying either only diffraction or reflection zone members any focallength, i. distance between the microphone and the center of the centralzone may be used. If this focal length is designated as F, the distancebetween the focal point to the inner and outer boundaries of the secondzone would be, respectively, F+/\/2 and F and A and the distance fromthe focal point to the outer boundary of the third zone would be F+3\/2.

In the embodiment of this invention illustrated in Fig. 4, themicrophone [2, which in this case preferably has a planar front, ismounted at the center of a circular reflecting member M, for example ofmetal. The reflecting member is provided with a central circular face 15of 1 wavelength in diameter, an annular recess l6, /4 wave-length deepand wave-length wide, and an outer annular surface I! also /2wave-length wide, the surfaces l5 and I1 and the recess l6 beingcoaxial. The effects of the central zone l5 and the third zone I! uponthe microphone I2 are in phase. The recess l6 corresponds to the secondzone and because of its wave-length depth produces a phase reversal ofthe waves entering thereinto so that in the plane of the surfaces l5 andI1 the resultant pressure due to the second zone is zero. Hence, thesecond zone is effectively self-canceling with respect to its effectupon the microphone.

Although in the construction illustrated in Fig. 4, three zones areinvolved, it will be understood that a greater number may be employed,the even-numbered zones being recesses wavelength deep and /2wave-length wide and the odd-numbered zones being /2 wave-length wide.

Although the invention has been described with 5. Particular reference.to microphone "devices, it may be embodied: also in receivers. inwhichcase a. suitable telephone receiver would be employed in place ofthe microphone l2. Also,'it will be understood that the, invention maybe embodied in devices suitable for submarine signaling as well as forsound propagation and reception in air. It will be understood furtherthatthe several devices disclosed and described are illustrative andthat various modifications may be made therein without departing fromthe scope and spirit of this invention as defined in the appendedclaims. Y

What is claimed is;

1. An acoustic device for translating signals within a band offrequencies comprising means defining a fl tin surface of a diametersufficient to embrace several concentric interference zonescorresponding to the mean wave-length of frequencies in said band, asound translatin device mounted at substantially the focal point of saidzones, and means for substantially preventing the second interferencezone from affecting said translating device.

2. An acoustic device in accordance with claim 1 wherein said preventingmeans comprises an annular masking member lying in a plane parallel tosaid surface and opposite said second interference zone thereof.

3. An acoustic device in accordance with claim 1 wherein said soundtranslating device is mounted at substantially the center of saidsurface and which comprises 'means bounding an annular recess conformingto said second interference zone and having a depth of substantially ythe mean wave-length of signals in said band.

4. An acoustic device for translating signals within a band offrequencies comprising means bounding a circular plane area of a'diameter sufficient. to include several concentric interference zonescorresponding to the mean wavelength of frequencies in said band, asound translatln device mounted at substantially the focal point of saidzones and to one side of said plane, and means masking said soundtranslating device from the second interference zone, said soundtranslating device being exposed to at least the central and thirdinterference zones.

5. An acoustic device for translating signals within a band offrequencies comprising means bounding a circular opening, and a soundtranslating device mounted to one side of and coaxial with said opening,said sound translating device being spaced from the center of saidopening a distance substantially equal to n)\ 3 where x is the meanwave-length in said band and n is an integer, said opening having adiameter of substantially 6. An acoustic device for translating signalswithin a band of frequencies comprising a sound translating device, zoneplate means in coaxial relation with said sound translating device, andmeans for substantially preventing wave pressures at the secondinterference zone from affecting said sound translating device.

'7. An acoustic device for translatin signals within a band offrequencies having a mean wave-length comprising means bounding acircular plane including several concentric circular zonal areas, asound translating device coaxial with said areas and so positioned withrespect thereto that the distance between said sound translating deviceand any point in the central zonal area. is between substantially saidwavelength and substantially /2 said wave-length, the zonal area nextadjacent said central area having such width that the distance betweensaid sound translatin device and any point in said next adjacent area isbetween substantially said wavelength and substantially /2 saidwave-length, and means for preventing pressures at said next adjacentarea from affecting said sound translating device.

8. An acoustic device for translating signals within a band offrequencies having a mean wave-length comprising a cylindrical memberhaving at one end a circular opening coaxial therewith and of. adiameter of substantially 1.7 times said wave-length, and a soundtranslating device within said cylindrical member in axial alignmentwith said opening and spaced ef fectively said wave-length from thecenter of said opening.

9. An acoustic device for translating signals within a band offrequencies comprising a cylindrical member closed at one end by asubstantially plane reflecting surface and having at the other end anopening coaxial therewith and of a diameter of substantially where A isthe mean. wave-length of the frequencies in said band and n is aninteger, said member having a depth of substantially within said member,substantially mid-way between the ends thereof and coaxial with saidopening.

11. An acoustic device for translating signals within a band offrequencies having a mean wave-length comprising a reflecting memberhaving a surface divided into a series of concentrio zonal areas, thecentral zonal area having a diameter substantially equal to saidwavelength and each of the other of said zonal areas being of a widthsubstantially equal to said wave-length, the zonal area next adjacentsaid central area being recessed and having a depth of substantially A;said wave-length, and a sound translating device mounted atsubstantially the center of said central zonal area.

12. An acoustic device for translating signals within a band offrequencies having a mean wave-length comprising a sound translatingdevice, reflecting zone plate means adjacent and coaxial with said soundtranslating device and having a diameter sufficient to include severalinterference zones corresponding to said wavelength, and means maskingsaid sound translating device from both the second diffraction andsecond reflecting interference zones.

13. An acoustic device for translating signals within a band offrequencies comprising a cylindrical member of a diameter sufiieient toinclude several interference zones corresponding to the mean wave-lengthin said band of frequencies, said member being closed at one end andopen at the other and having a depth substantially equal to times saidwave-length, n being an integer, a sound translating device within saidmember, on the axis thereof and substantially mid-way between the endsthereof, and an annular masking member coaxial with said cylindricalmemher and substantially in the plane of the open end thereof, saidannular member having an inner diameter of substantially x ffl i timessaid wave-length and an outer diameter of substantially times saidwave-length.

14. An acoustic device for translating signals within a band offrequencies having a mean wave-length comprising a cylindrical member ofa diameter sufficient to include several interference zonescorresponding to said wave-length, said member being closed at one endand open at the other and having a length approximately wave-lengthcomprising a zonal system including a reflecting member of a diametersufiicient to include several interference zones corresponding to saidwave-length, an annular imperforate member parallel to and coaxial withsaid reflecting member and corresponding to the second interferencezone, and a sound translating device mounted at substantially the focalpoint of said system.

16. An acoustic device for translating signals within a band offrequencies comprising an annular zone plate, a sound translating devicecoaxial with and to one side of said zone plate, said zone plate beingso constructed and arranged that the distance between said soundtranslating device and the inner edge of said zone plate issubstantially and the distance between said sound translating device andthe outer edge of said zone plate is substantially F+ where F is thedistance between said sound translating device and the center of saidzone plate and A is the mean wavelength of frequencies in said band.

EDMOND BRUCE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,572,387 Harrison Feb. 9, 19261,755,636 Dubilier Apr. 22, 1930 1,843,524 Stenger Feb. 2, 19321,969,704 DAlton Aug. 7, 1934 2,135,840 Pfister Nov, 8, 1938 2,216,949Kellogg Oct. 8, 1940 2,228,024 Abrahams Jan. 7, 1941

